SELFOSS, Iceland — Geoscientists have exposed another assault on Earth’s protective ozone layer — not by manufactured chemicals, but by gas ejected in the blasts of huge volcanic eruptions.

A new study shows that volcanic rocks in Nicaragua contain bromine, an element known for speeding ozone’s destruction in the upper atmosphere. When magma erupted to form those rocks, scientists say, it also released huge amounts of bromine into the air — enough to destroy large parts of the ozone layer f...